1. Field of the Invention
The present invention relates to injection compression molding method and machine of a lens for injecting and compressing thermoplastic resin to mold a lens. More specifically, it relates to an injection compression molding method and an injection compression molding machine of a lens capable of reducing sink mark and strain, restraining strain of molding article, and easily changing the thickness of the molding article, which is suitable for molding a spectacles-lens.
2. Description of Related Art
Conventionally, an injection compression molding method is known as a method for molding spectacles lenses using thermoplastic resin.
In the injection compression molding method, in order to compensate shrinkage of molten resin to obtain uniform and high shape accuracy, molding die is clamped while retaining compression margin inside the spectacles-lens molding cavity, the molten resin is injected and filled in the spectacles-lens molding cavity, the molding die is compressed by the compression margin and the molten resin is cooled to obtain the spectacles-lens (see Japanese Patent Laid-Open Publication No. Hei 9-277327 and Japanese Patent Laid-Open Publication No. Hei 9-216263).
Since a meniscus lens used as a spectacles-lens is a thick molding article having thickness difference between the central portion and peripheral portion thereof, the cooling time has to be taken long in order to restrain the sink mark, shrinkage deformation, internal strain etc, which lengthen molding cycle.
Japanese Patent Laid-Open Publication No. Hei 9-234774 discloses a cooling time of 230 seconds for molding a minus lens (a lens having thicker peripheral portion than the central portion) of 76 mm diameter and lens power of −4.00 D (diopter).
In the above cooling step, the molten resin compressed in the molding die clamped by a predetermined clamping force loses resin pressure thereof inside the molding die in accordance with molding shrinkage due to cooling. The resin pressure gradually decreases in accordance with the progress of cooling and, after completion of cooling, the molding article is released from the molding die during ejecting step.
In order to obtain a lens with high accuracy, it is important to restrain the sink mark, shrinkage deformation, internal strain and deformation during die-releasing step as well as raising the transferability for accurately transferring the concave and convex shape of the spectacles-lens molding cavity to the lens. Accordingly, a cooling step for uniformly and sufficiently cooling the entire molten resin filled in the spectacles-lens-molding cavity is required.
In the conventional injection compression molding method, the molten resin is cooled for a predetermined time while keeping the pressure applied to the resin at a predetermined level during the cooling step, and the molding die is rapidly parted to reduce the pressure applied to the resin instantly. Accordingly, deformation is likely to be caused on the molding article during the die-releasing step if the cooling time is not sufficient.
Further, there can be partial parting (peeling) between the dies and the molding article according the manner for opening the dies during the die-releasing step, which causes parting failure of so-called “separation”.
Further, the lens thickness has to be changed in accordance with lens characteristics, e.g. lens power (such as spherical power and astigmatic power) in molding a spectacles-lens including finished lens and semi-finished lens.
In the conventional method, since the lens is obtained by compressing the molding die for a predetermined compression margin, i.e., since the cavity is reduced to a position for giving a desired thickness of final product, in order to change the thickness of a lens, a molding die having a cavity of final thickness of molding article has to be prepared for each lens and the molding article has to be changed in accordance with the lens to be molded.
Accordingly, a plurality of types of molding dies have to be prepared and much production cost is necessary, and additional work is required for exchanging the molding die, thereby deteriorating production efficiency.